J . Am. Chem. SOC.1985, 107, 47-52 Tris buffer described (containing ethanol and EDTA). Quantitative Thin-Layer Chromatography. After qualitative thin-layer chromatographic analysis was made (iodine detection), silica gel containing adsorbed lipid was removed from appropriate zones and analyzed directly for phosphorus content. Procedures used were similar to those previously de~cribed,~' except a larger volume of magnesium nitrate solution (ca. 0.20 mL) was employed, and the derivatized mixture was filtered to remove silica gel prior to UV analysis. Vesicle Polymerization. Typically, 7 pL of 30% H 2 0 2(20 equiv) was added to a 1.0-mL dispersion of l a containing 2.0 mg of lipid, and the resulting dispersion was heated under a nitrogen atmosphere for 3 h at 40 OC. Thin-layer chromatography showed a single spot at the origin (iodine). The dispersion was then dialyzed against 100 mL of doubly distilled water for 18 h at room temperature in order to remove excess hydrogen peroxide. Quantitative analysis for thiol content indicated that ca. 5% of the thiol groups remained. Vesicle Depolymerization. Typically, a 1 .O mL of polymerized dispersion of l a (2.0 mg of lipid) that had been dialyzed to remove excess hydrogen peroxide was purged with a stream of nitrogen and mixed with 60 mg (0.388 mmol) of dithiothreitol. The resulting dispersion was purged with nitrogen for 5 min and heated in a water bath for 1.5 h at 50 O C . Qualitative analysis by TLC indicated substantial regeneration of la; R,, = 0.22, solvent C, positive thiol test. Quantitative phosphorus
47
analysis revealed a 63% yield of regenerated l a . Freeze-Fracture Electron Microscopy. Samples of sonicated and polymerized dispersions of l a (20 mg/mL) were frozen in liquid nitrogen, fractured with etching by using a Balzer apparatus, and replicated with Pt/C by using standard procedure^.^^ Glycerol (5%) was added to prevent freeze damage. Electron micrographs of freeze-fractured samples were obtained by using a Philips 201 instrument.
Acknowledgment. W e a r e grateful t o Tracy Handel (California Institute of Technology) for recording the freeze-fracture electron micrograph of polymerized vesicles of la. W e a r e also grateful t o D r . Jitender Khurana for valuable technical assistance. Registry No. la, 87050-11-1; Ib, 93404-44-5; IC, 93404-45-6; 2, 93404-46-7; 3a, 87050-15-5; 3b, 93404-47-8; 4a, 87050- 14-4; 4b, 93404-48-9; 5, 93404-49-0; 6, 93404-50-3; GPC, 283 19-77-9; dithreitol, 3483-12-3; ethyl ethanethiosulfinate, 18542-39-7; hydrogen peroxide, 7722-84-1; 11-mercaptoundecanoic acid, 71310-21-9; 1-octanethiol, 11 1-88-6.
(38) Ververgaert, P. H. J. Th.; Elbers, P. F.; Luitingh, A. J.; Van de Berg,
H.J. Cytobiology 1972, 6, 86.
Electron-Relay Chain Mechanism in the Sensitized Photoisomerization of Stilbazole Salts in Aqueous Anionic Micelles Katsuhiko Takagi,+ Keitaro Aoshima,+ Yasuhiko Sawaki,*t and Hiizu Iwamura*$ Contribution from the Department of Applied Chemistry, Faculty of Engineering, Nagoya University, Furo-cho, Chikusa-ku, Nagoya 464, Japan, and the Institute for Molecular Science, Myodaiji-cho, Okazaki 444, Japan. Received April 30, 1984
Abstract: Cis to trans isomerization of N-methyl-4-(j3-styryl)pyridinium halide (4-MSPX, X = I or C1) via electron-transfer sensitization by R ~ ( b p y ) , ~has + been studied for some micellar systems under the illumination of 468 f 5 nm light. Efficiencies for the isomerization (4), were markedly enhanced on addition of anionic surfactans; e.g., the quantum yields in the presence of sodium dodecylsulfate (SDS) reached the maximum value of 64 f 2, which is about 100-fold as much as those without SDS. The value is comparable to the aggregation number, NA,of the S D S micelle, involving the pyridinium ions equal to SDS molecules. Similar agreement between N A and &t. values was observed with micelles of other anionic surfactants CH,(CH2),0SO3Na ( n = 9, 11, 13). It is postulated that an electron-relay chain mechanism is operative on the anionic micellar surface for the sensitized isomerization of pyridinium ions attached electrostatically. The line-width measurements of 23Na N M R indicated that 4-MSP ion effectively substitutes the Na+ ion adsorbed on SDS micelles. An overwhelming high adsorptivity of 4-MSP ion on anionic micelles over N a + ion was also evidenced by effects of various added salts in the reaction mixture.
An efficient electron transfer between donor D * a n d acceptor A (eq 1) in micellar systems is the subject of recent interest. T h e
-
+A D+ + A--
D*
D+ + A-
(l)
+A
(2)
D
heterogeneous electrostatic field formed by surfactant molecules is considered t o r e t a r d t h e back transfer of a n electron once generated (eq 2), enhancing t h e efficiency of net charge separation.] Thus, much attention has been focused on redox reactions utilizing anionic micellar systems in relation t o solar energy storage.* Interaction of a hydrated electron with surfactantsolubilized benzene, for example, showed significant micellar effects;, t h e electron addition t o t h e benzene molecule was retarded b y sodium dodecyl sulfate (SDS) b u t enhanced b y t h e cetyltrimethylammonium bromide ( C T A B ) micelle. T h e t w o opposite effects suggest t h a t t h e penetration of e,; into the micelle interior Nagoya University. Institute for Molecular Science.
0002-786318511507-0047$01.50/0
of solubilized benzene is hindered by t h e negatively charged SDS micellar surface b u t enhanced by cationic CTAB m i ~ e l l e . ~ A s a n alternative effect of micellar systems, anionic micelles a r e expected t o force cationic substrates t o aggregate on its negatively charged surface. T h e r e a r e few studies forcused on the reactions of substrates adsorbed on micellar surfaces. Electron (1) (a) Wallace. S. C.: Gratzel. M.: Thomas. J. K. Chem. Phvs. Lett. 1973. 23, 359. (b) Thomas, J. K.; Piciulo, P.'L. J. Am. Chem. SOC.1979, 101, 2502: (c) Schmehl, R. H.; Whitten, D. G. Ibid. 1980, 102, 1938. (d) Meisel, D.; Matheson, M. S.; Rabani, J. Ibid. 1978, 100, 117. ( e ) Waka, Y . ;Hamamoto, K.; Mataga, N. Chem. Phys. Lett. 1978, 53, 242. ( f ) Katusin-Razem, B.; Wong, M.; Thomas, J. K. J . Am. Chem. SOC.1978, 100, 1679. (g) Tsutsui, Y.; Takuma, K.; Nishijima, T.; Matsuo, T. Chem. Lert. 1979, 617. (h) Brugger, P.-A.; Gratzel, M. J . Am. Chem. SOC.1980, 102, 2461. (2) (a) Calvin, M. Acc. Chem. Res. 1978, 11, 369. (b) Moroi, Y . ;Braun, A. M.; Gratzel, M. J . Am. Chem. Soc. 1979,101, 567. (c) Kano, K.; Takuma, ~
I
,
,
K.; Ikeda, T.; Nakajima, D.; Tsutsui, Y . ;Matsuo, T. Photochem. Photobiol. 1978, 27,695. (d) Willner, I.; Ford, W. E.; Otvos, J. W.; Calvin, M. Nature (London) 1979, 280, 823. (3) Fendler, J. H.; Patterson, L. K. J . Phys. Chem. 1970, 74, 4608. (4) Fendler, E. J.; Day, C. L.; Fendler, J. H. J . Phys. Chem. 1972, 76, 1460.
0 1985 American Chemical Society
48 J . Am. Chem. Soc., Vol. 107, No. 1. 1985 transfer between a n electron donor a n d an acceptor adsorbed electrostatically o n t h e micellar surface is a n interesting b u t unsettled a r e a in relation to many electron transport phenomena, e.g., photosynthesis a n d solar energy storage. We wish t o report h e r e our findings t h a t aggregation of a positively charged electron acceptor on anionic micelles may lead t o a highly efficient chain reaction initiated by a n electron transfer f r o m t h e excited donor molecule. Thus, Ru(bpy),2+-sensitized isomerization of cis-N-methyl-4-(P-styryl)pyridiniumhalide (cis-4-MSPX; X = CI or I) t a k e s place q u i t e efficiently in t h e presence of anionic surfactants with the q u a n t u m yields as high as 50-100. T h e high efficiency is probably based on an electron-relay chain isomerization of cis-4-MSPX aggregates on t h e anionic micelles. This is, t o our knowledge, t h e first example of chemical reactions of aggregates on micelles by a n electron-relay chain mechanism.
Takagi et al.
60
w U
K3I
40-
20
-
Experimental Section Materials. Tris(2,2'-bipyridine)ruthenium dichloride [ R ~ ( b p y ) ~ ] * + (Cl-)2,5 zinc tetraphenylporphyrin (Zn(TPP)),, zinc tetrasodiotetrakis@-sulfopheny1)porphyrin (Zn(TPP)S),7 and zinc tetrakis(4-N-methylpyridinium)porphyrin tetraiodides were prepared according to the literature. The complexes prepared were satisfactorily pure (UV and N M R spectroscopically) and dried at 100 OC for 1 h prior to use. cis-Nmethyl-4-(P-styryl)pyridinium chloride (cis-4-MSPX, X = CI) was prepared by the halogen ion exchange of cis-N-methyl-4-(/3-styryl)pyridinium iodide (cis-4-MSPX, X = I)9 through a column packed with ion-exchange resin (Amberlite IRA-400, Rohm & Haas Co.). cis- and trans-N-methyl-3-(P-styryl)pyridinium iodides (cis-and trans-3-MSPX, X = I) were obtained by the methylation of cis- and trans-3-P-stilbazoles,1° respectively, with Me1 in refluxing benzene. The cis and trans isomers melt and decomose at 133-137 and 222-224 OC, respectively. The content of the iodide ion was determined after the oxidation with 3% aqueous H 2 0 2followed by the extraction of iodine with CHCI,(X,,, 535 nm)." The results showed that -97% of the iodide ion was substituted for chloride ion in the case of the exchange of the counterion of cis-4MSP ion. Sodium decyl sulfate (SDeS) and tetradecyl sulfate (STS) were prepared by the reaction of the corresponding alcohols with chlorosulfonic acid followed by neutralization with aqueous NaOH.I2 The crude sulfates were purified by extraction of the organic impurities with refluxing n-hexane and recrystallization from H 2 0 . Sodium dodecyl sulfate(SDS) was of commercial extra pure grade (Tokyo Kasei Co.) and dried under vacuum before use. Apparatus. Absorption spectra were recorded on a Hitachi ultraviolet spectrophotometer (Model 124). Proton N M R spectra were done on a Hitachi R-24B N M R spectrometer using dimethyl-d, sulfoxide or CDCI, containing TMS. 23Na N M R spectra were taken on a Varian FT-8OA N M R spectrometer (21.039 MHz) at 30 f 0.5 OC. The line widths at half-height were read directly from the spectral chart. The reference used as a 0.02 M aqueous solution of NaI. Luminescence spectra were recorded on a Hitachi 650- 10 fluorescence spectrophotometer in appropriate solutions. Reduction potentials were taken on a Yanagimoto P 1 100 polarographic analyzer. Electrochemical analyses were performed using a mercury electrode in acetonitrile containing M substrates and 0.05 M tetraethylammonium perchlorate (TEAP) as an electrolyte. HPLC analyses were made on a Jasco Twinkle HPLC instrument equipped with a UV detector (345 nm). The HPLC analysis was done using an ODS column (Fine Si1 C,,-IO; Jasco) and a mixture of 0.1 M aqueous NaCl and MeOH (1:4) as an eluent. Cmc's were determined by a Shimadzu du Nouy type tensiometer. Irradiation. Continuous illumination experiments were carried out in the same cell of a fluorescence spectrophotometer equipped with a 150-W xenon lamp and a monochromator. In a typical run, an aqueous solution (3 mL) of Ru(bpy)32+sensitizer (3.1 X lo-' M), cis-4-MSPI (0.01 M), and SDS (8.5 X lo-' M) was transferred into a 1 X 1 cm glass cuvette equipped with a serum stopper, deaerated by flushing with oxygen-free argon, and irradiated continuously (5) Burstall, F. H. J . Chem. SOC.1936, 173. (6) Fuhrhop, J.-H.; Smith, K. L. In "Porphyrins and Metalloporphyrins"; Smith, K. M., Ed.; Elsevier: Amsterdam, 1975; p 769. (7) Srivastava, T. S.; Tsutsui, M. J . Org. Chem. 1973, 38, 2103. (8) Fleischer, E. B. Inorg. Chem. 1962, I , 493. (9) Takagi, K.; Ogata, Y . J . Org. Chem. 1982, 47, 1409. (IO) Clarke, F. H.; Felock, G. A,; Silverman, G. B.; Watnick, C. M. J . Org. Chem. 1962, 27, 533. (11) Ishibashi, M.; Sahara, R. Nippon Kagaku Zasshi 1940, 61, 513. (12) Maurer, E. W.; Stirton, A. J.; Weil, J. K. U.S.Patent 3 133946; Chem. Abstr. 1964, 61, 2973a.
0
[m -4-MSP 1I/ I
1,5
1.0
0.5
SDSl
Figure 1. Effect of molar ratios of cis-4-MSPI/SDS on the quantum yields (&J of the photoisomerization of cis-4-MSPI in water at 20 i 2 O C : [Ru(bp~)3~'],8.6 X M; [SDS], 6.0 mM; irradiated at 468 f 5 nm. at 468 f 5 nm. The formation of trans-4-MSPI was monitored by HPLC analysis of an aliquot taken at appropriate time intervals. The quantum yields were determined by potassium ferrioxalate actin01netry.I~ The incident photoflux amounts to 9.5 X einsteins/L min.
Results and Discussion We have already shown t h a t t h e photochemical cis-to-trans isomerization of N-methyl-4-((3-styryl)pyridiniumhalide (4M S P X ) is induced via a one-electron reduction by visible-light sensitizer^.^ Irradiations of cis-4-MSPI in t h e presence of Ru(bpy)j2+ or tetraphenylporphyrin complexes result in t h e pred o m i n a n t formation of the t r a n s isomer (289%) a t t h e photostationary s t a t e (pss). T h e r a t e constants for t h e quenching of the fluorescence of the sensitizers by various MSPI's a r e correlated with their reduction potentials, showing t h a t t h e excited dyes initiate t h e isomerization by a n electron transfer to cis-CMSPI a s depicted i n eq 3.9 T h e isomerization is quite in contrast t o
Ph
PR
cis-4-MSP ion
d
" \
h
(3)
Ph
trans-4-MSP ion ordinary triplet-sensitized isomerizations leading t o cis-rich mixtures. The trans-predominant photoequilibria of MSP ions reflect the almost exclusive formation of t h e m o r e stable t r a n s isomer f r o m the pyridinyl radical intermediate ( P R ) .
Cis-to-Trans Isomerization to cis -4-MSPI by Ru(bpy)32+*in Micellar Solutions. T h e q u a n t u m yields for t h e photochemical + reisomerization of cis-4-MSPI sensitized by R ~ ( b p y ) , ~were m a r k a b l y enhanced by t h e addition of sodium dodecyl sulfate (SDS) in comparison to those in homogeneous aqueous solutions. Mostly, t h e isomerization was carried out under argon in aqueous solutions containing 6-10 mM SDS, t h e critical micelle concentration (cmc) of which was quite below 0.0081 M. Figure 1 shows t h a t t h e efficiency for t h e isomerization increased steadily with t h e increasing ratios of [cis-4-MSPI] to [SDS] a n d reaches to a limiting value of 6 4 f 2 a t t h e ratio around 1.O. Similarly, t h e (13) Hatchard, C. G.; Parker, C. A. Proc. R . SOC.London, Ser. A 1956,
,4235, 518.
Photoisomerization of Stilbazole
J. Am. Chem. Soc., Vol. 107, No. 1, 1985
0
50
100
150
49
200
ISDSI/I Ru ( ~ D Y1'23 ) 0
50
150
100
250
[e-4-MSPII/[Ru( ~ D Y ) ~ + I
Figure 2. Effect of c i s - 4 - M S P X / R ~ ( b p y ) ~on~ +the quantum yields (&+) of the photoisomerization of cis-4-MSPI in water at 20 f 2 OC: M; [SDS] = [cis-4-MSPI]; irradiated at 468 [R~(bpy)~*'], 8.6 X f 5 nm. Table I. Limiting Quantum Yields (@-,),,,ax for the Isomerization of cis-4-MSPI in Various Surfactant SvstemP
aggregation no.b surfactant sodium dodecyl sulfate (SDS) sodium decyl sulfate (SDeS) sodium tetradecyl sulfate
NA'
( * - t L X
Na
0.58 f 0.03 64 f 2 (63)c
62d
70e
49 f 1
50d
60'
86 f 4
82e
(SW
SDS microemulsion' 98 f 9 1008 113e dodecyltrimethylammonium 0.73 f 0.05 50d bromide (DTAB) "Conditions: [ R ~ ( b p y ) ~ ~ 7.7 ' ] , X IOd to 9.7 X M; [surfactant], 1.8-6.0 mM; [cis-4-MSPI], 2.0-7.2 mM. Irradiation was carried out under argon with light at 468 f 5 nm at 20 2 'C. bNAoand NA mean aggregation numbers determined in the absence of and in the presence of 0.01 M benzyltrimethylammonium bromide (BTAB). The NA values represent more accurately the true aggregation numbers in the presence of cis-4-MSPI. 'Using cis-4-MSPX (X = C1) instead of cis-4-MSPI. Reference 16a. Calculated by a luminescence probe.I9 /A mixture of SDS (10 mM), amyl alcohol (61 mM), and dodecane (16 mM) in water. gReference 16b.
*
effect of molar ratios of [~is-4-MSPI]/[Ru(bpy),~+] on the efficiency resulted in the same limiting quantum yield when the ratios were larger than around 70 (Figure 2). Thus, the optimum reaction system to attain the maximum quantum yield appears to consist of ca. 70 molecules of cis-4-MSPI and SDS per one molecule of R ~ ( b p y ) , ~ + . The sensitized isomerization was enhanced by anionic micelles more than 100 times as compared to that in the homogeneous aqueous solution. In the micellar reactions, no effect of counterion, X-, in cis-4-MSPX was observed; the value for cis-4-MSPCI was identical with that of cis-4-MSPI (Table I). In contrast to the acceleration by anionic micelles, cationic micelles such as DTAB (dodecyltrimethylammonium bromide) had little effect on the isomerization. These results clearly suggest that the electrostatic anionic field is essential for the dramatic enhancement of the quantum yields. Quenching of Luminescence of Ru(bpy)32+*with cis-6MSPI in Micellar Solutions. The effect of SDS micelles on the luminescence of R ~ ( b p y ) ~ ~ is +noteworthy. * The luminescence quenching constants (Le., kqr)by cis-4-MSPI as determined from the Stern-Volmer plots were maximum at the SDS/Ru*+ ratio
Figure 3. Effect of molar ratios of SDS/Ru(bpy)32Con the fluorescence quenching of R ~ ( b p y ) ~ ~by ' * cis-4-MSPI at 20 2 'C: [R~(bpy),~'], 3.2 X M; [SDS], 0-7.0 mM; 7 , lifetime of Ru(bpy),*'*. The kq7 values were determined by the Stern-Volmer plots with [cis-4-MSPI] = 0-2.0 mM. Open circles were obtained from the slopes of the linear relationship between I o / I and [cis-MSPX],
of ca. 63 (Figure 3). Namely, the quenching was the most efficient when just one molecule of the Ru2+complex was attached to each SDS micelle. The lower k q T values at SDS/Ru2+ < 60 reflect the bimolecular quenching of Ru2+*with ground-state Ru2+ since one micelle involves several molecules of Ru2+ions under these conditions. In general, deactivation of Ru2+*may be expressed as in eq 4. The third term in eq 4 is noted to play an -d[Ru2+*]/dt = kl[Ru2+*] + kz[Ru2+*]2+ k3[Ru2+*][Ru2+](4) important role for the deactivation in case of weak and steady-state ill~mination.'~ Above the SDS/Ru2+ ratio of 60, the apparent quenching by cis-4-MSPI decreased by the increasing ratio. This is responsible for the net decrease of cis-4-MSPI molecules per one molecule of the Ru2+complex, since the number of micelles involving only cis-4-MSPI but not Ru*+ molecules increases. Thus, the quenching studies lead to the same conclusion that the most efficient sensitization consists of one molecule of Ru2+ per an anionic micelle. Effects of Anionic Micelles of CH3(CH2),0S03Na(n = 9, 11, and 13) on the Isomerization. The above results on the SDS micelle suggest that the optimum quantum yield can be obtained when an anionic micelle contains one Ru2+ complex and cis-4MSPI molecules approximately equal to the aggregation number ( N A o ) ,where NAodenotes the number of surfactant molecules for a micelle without any additives. Similar results were obtained with other anionic micelles and a microemulsion system for the Ru2+-sensitized isomerization of cis-4-MSPI. As shown in Table I, the resulting quantum yields for the isomerization are in good agreement with the reported NAo value^'^*'^ of corresponding anionic surfactants. The detailed discussion about &.r and NA0's will be given later in mechanistic considerations. Cationic micelles (e.g., DTAB) did not show any accelerating effect. Effects of Added Salts on the Isomerization. It is known that added salts tend to increase the aggregation number of SDS and to decrease the c ~ c . ~ ' J *In the present photoisomerization of cis-CMSPI, the addition of salts to the SDS micellar solution is expected to exert a significant effect. Sodium chloride, benzyltrimethylammonium bromide (BTAB), and tetramethyl(14) Matsuo, T. Kagaku Sosetsu 1982,No. 33, 21 1. (15) These a regation numbers, NAo,are the reported ones in the absence of the additives? (16) (a) Fendler, J. H.; Fendler, E. J. In "Catalysis and Macromolecular Systems"; Academic Press: New York, 1975; p 20. (b) Almgren, M.; Grieser, F.; Thomas, J. K. J . Am. Chem. SOC.1980, 102, 3188. (17) Mysels, K. J.; Princen, L. H. J . Phys. Chem. 1959, 63, 1696. (18) Almgren, M.; Swarup, S. J. Phys. Chem. 1983, 83, 876.
50 J . Am. Chem. Soc., Vol. 107, No. 1 , 1985
Takagi et al.
Table 11. Effect of Added Salts on Quantum Yields and N A and SDS" [Saltladded, 0
NAb
@ e t
64
62
0.05 0.10 0.20 0.30
NaCl 62 53 34 30
68 75 87 101
0.005 0.010 0.020 0.030
BTABE 51 39 33 25
63 70 71 75
a,
n
.-2
n
n
TAB^ 0.030 59 0.10 38 72 "Conditions: [ R ~ ( b p y ) ~ ~3.1 + ] X, M; [SDS], 8.5 mM. Irradiation, 468 5 nm under argon at 20 2 "C. *Aggregation numbers in the presence of salts as measured by the method of Turro et al. (ref 19). Benzyltrimethylammonium bromide. dTetramethylammonium bromide.
*
.8
0
Table 111. Electron Transfer Efficiency from Excited Ru(bpy),*+ to Acceptor" acceptor
solvent
apt
% trans AG,~ at pss kcal/mol remarks
cis-4-MSPI H,O 0.58 89 -8.6 C cis-4-MSPI S D S / H 2 0 66 98 (-8.6) d trans-4-MSPI H 2 0 89 -8.6 H,O 0.047 -5.3 d cis-3-MSPI cis-3-MSPI SDS/H20 3.1 45 (-5.3) d cis-stilbene CH3CN 5 +17.5 e "Conditions: [ R ~ ( b p y ) ~ ~3.2 +]X , M; [olefin], 4.1 mM; [SDS], 8.5 mM. Irradiation at 468 i 5 nm under argon at 20 f 2 OC. bThe free-energy change in the electron-transfer process between excited Ru(bpy)3'+ and acceptor. Electron-transfer-induced isomerization is operative (ref 9). dThe present work. @Energy-transfer-induced isomerization is operative (ref 26).
s'..,
10
20
30
[Saltl/[c~-4-MSPII Figure 4. Dependence of quantum yields (4-J on added salts for the isomerization of cis-4-MSPI: ( 0 ) for addition of NaC1; (a) for benzyltrimethylammonium bromide (BTAB). The reaction conditions are identical with those in Table 11.
ammonium bromide (TAB) were examined. Table I1 summarizes the values together with the aggregation numbers ( N A )of SDS in the presence of the salts which were measured according to the procedure of Turro et al.19 All the salts examined retarded the isomerization significantly. The least soluble BTAB retarded the reaction 10 times as efficiently as NaCl or TAB, implying that BTAB tends to adsorb most effectively on the anionic micellar surface of SDS and hence decrease the adsorption sites available for cis-4-MSPI. Thus, 0.3 M NaCl increased the NAovalues of SDS almost twice but, at the same time, reduce the &t. value to a half when 0.01 M cis-4-MSPI was used. In other words, the adsorption equilibrium constant for the cis-4-MSP ion appears to be 20 times as high as that for the sodium cation. Interestingly, a hydrophobic, i.e., less soluble, BTAB cation is more susceptible to absorption to the SDS micelle. It is clearly shown in Figure 4 that the hydrophobic salt adsorbs on the micellar surface effectively, and its adsorption constant is approximately the same with cis-4-MSPI. It is reasonable that cationic ions of similar hydrophobic groups possess the adsorption constants of the same order. NMR Study of 23NaIon in Aqueous SDS. It is apparent in Figure 4 that the sodium ion adsorbs to the SDS micelle 20 times less effectively than cis-4-MSPX. It is then expected that the ~
~~~
(19) Turro, N . J.; Yekta, A. J . Am. Chem. SOC.1978, 100, 5951.
sodium ion would be easily detached on addition of cis-4-MSPX. This could most directly be measured by 23NaN M R relaxation. Since the 23Na nucleus possesses an electric quadrupole Q, its relaxation rates are governed by the coupling between Q and the electric field gradient at the nucleus. The latter should be quite small for the free Na+ and substantial when the ion is paired to the counteranion. Therefore, the slow and fast relaxation rates would mean the free and ion-paired Na+'s, respectively.20 Figure 5 shows the dependency of the observed line widths of the 23Na N M R signal on the concentration of cis-4-MSPX in the presence of SDS. Under extreme narrowing conditions, the line width represents the apparent rate (KbSd) which is the average relaxation rate of free (Rf) and adsorbed ions (R,) as indicated by eq 5.20
Here, ct and, c denote the total concentration and cmc of SDS, respectively, and 6 means the degree of adsorption on the micelle. The results in Figure 5 clearly indicate that the addition of cis4-MSPX promotes the release of adsorbed sodium ion, implying the concurrent adsorption of cis-4-MSP pyridinium ion. In other words, the P value of Na' was steadily decreased by the addition of cis-4-MSPX2' and was estimated t o be 3.8% at t h e cis-4MSPX/SDS ratio of 0.5, which is significantly lowered compared to the reported @ value of 73% in the absence of ~ i s - 4 - M S p X . ~ " ~ ~ The adsorbed sodium ion appears to be substituted almost completely with the pyridinium ion at the cis-4-MSP+/Na+ ratio of 0.5. The 23Na study supports the above-mentioned micelle (20) (a) Laszlo, P. Angew. Chem., Inr. Ed. Engl. 1978, 17, 254. (b) Gustavsson, H.; Lindman, B. J . Am. Chem. SOC. 1975, 97, 3923. (21) The cmc of SDS in the presence of 4.1 mM cis-4-MSPI was measured M. to be 9.1 X (22) Estimations of the micellar charge have been reported by several workers.23 The /3 values are scattered between 0.22 and 0.36, among which /3 = 0.27 has been assessed to be the most reliable by N i ~ h i k i d o . ~ ~ (23) Koshinuma, M.; Sasaki, T. Bull. Chem. SOC. Jpn. 1975, 48, 2755. (24) Nishikido, N. J. Colloid Interface Sci. 1983, 92, 588.
Photoisomerization of Stilbazole
J. Am. Chem. Soc., Vol. 107, No. 1 , 1985 51
model, consisting of one Ru2+complex and selectively adsorbed 4-MSP cation on the anionic micelle. Electron-Transfer Isomerization of Various Olefins. In Table I11 are summarized the Ru2+-sensitized cis-trans isomerization of various olefins. The SDS micellar system revealed the remarkable, enhancing effect on the quantum yields for the isomerization of MSPI's. Thus, the &t. for the Ru2+-sensitized isomerization of cis-4-MSPX in aqueous solution increased more than 100-fold by addition of SDS. Excited ruthenium complexes can induce the isomerization of olefins via an excited triplet sensitization and/or one-electrontransfer mechanisms. Sensitization by triplet energy transfer can induce isomerizations from both isomers (trans and cis), while one-electron-induced isomerizations lead to irreversible cis-to-trans isomerizations as a diagnostic index.9 Hence, the %trans at the pss would be close to 100% if only the electron-induced isomerization is operative. For the Ru2+-sensitization in homogeneous aqueous solution, the high trans/& ratio of 8.1 (Le., %trans -89%) at the pss indicates an almost but not exclusive occurrence of the electron-transfer mechanism. Interestingly, the %trans further increased to >98% at the pss in the SDS micellar solution, implying the electron-transfer chain isomerization on micellar surface and the negligibly small contribution of triplet sensitization. An enhancement (ca. 60-fold) of the dvt value by SDS was also observed for the 3-styryl analogue (cis-3-MSPI). The electron transfer from Ru2+*is expected to be diffusion-controlled judging from the exothermic free-energy change (Le., negative AG value) for cis-3- and -4-MSPX. The approximately 10-fold lower quantum yields for cis-3-MSPI as compared with the cis-4-isomer suggest the less efficient isomerization by the one-electron transfer. The less efficient and lower %trans of 45% at the pss for cis-3MSPX may reflect the lower e~othermicity.~~ It is understandable that the efficiency for electron transfer to the 3-isomer is less favorable on account of the lower exothermicity than that to the 4-isomer, while the triplet energy transfer from Ru2+*to MSPX is of similar efficiency between the 3- and 4-isomers because of the same ET value of ca. 50 kcal/moL9 That is, the Ru2+*-sensitized isomerization of cis-3-MSPX is implicated to take place, even in the SDS micellar system, via the triplet sensitization to a considerable extent. The isomerization of cis-stilbene via the electron-transfer mechanism is unfavorable because of the larger positive AG value; in fact, the %trans was only 5% at the pss as shown in Table cis-3- and -4-MSPX may be photoisomerized by direct irradiation at 285 f 10 nm, but the efficiency (Le., &-,) could not be enhanced by addition of SDS. This fact rules out the occurrence of the triplet energy induced chain isomerization of the stilbazole salts adsorbed on the SDS micellar surface. We have already shown that zinc tetraphenylporphyrin (Zn(TPP)) sensitizes the isomerization of stilbazoles by a one-electron t r a n ~ f e r .When ~ negatively charged Zn(TPP)S was used for the present sensitization, the accelerating effect of the SDS micelle (3-8 mM SDS) was very low, Le., by factors of 1.5-2. The result is easily understood since the coulombic repulsion does not allow Zn(TPP)S to adsorb on the anionic SDS micellar surface. Electron-Relay Chain Mechanism. All the above results indicate that it is essential for the efficient electron transfer between D* and A to come in contact with each other on the same micellar surface. A possibility that the present high quantum yields might be originated on intermicellar electron-exchange processes could not be substantiated on the basis of almost constant &+t values in the high concentration range of [cis-MSPX] as shown in Figure 2. The electron-relay chain reaction as depicted in Scheme I is (25) Another reason for the lower efficiency is probably the lower delocalization of the electron in the PR radical having the meta substitution. Me-N
